2014-03-02, 10-11 UTC

Supercooled liquid water

Moments only from „principal peak“; can change between liquid and ice peak depending on which one is stronger!

Characteristics of cloud water peak: If turbulence is low, peak is very narrow (narrow PSD), close to 0 m/s, often separated from faster falling small ice and aggregates. Can be used as tracer for estimating vertical air velocity.

Thickness of supercooled layer at cloud top (use the MDV switching artefact!) seems to be well correlated with LWP curve.

The RS-RH profile suggests a second possible layer of supercooled water at around 500m. Since turbulence increases drastically in the lowest 500m it is not easy to find a clear liquid peak here. Maybe the size and concentration of droplets is just to small/low to be detected…

Riming

Starting at the liquid layer we already find a peak between 0.5 and 1 m/s. This could be due to small ice and first aggregates or already rimed single crystals (hard to say without other parameters)

Just a few hundred meters below, the dominant peak is already between 1 and 1.7 m/s while we find a large area of different ice velocities. Those large Doppler velocities indicate additional riming has happened during the last few hundred meters

Finally, at 500m, the main peak speeds up to 1.5 to 2 m/s, a clear riming feature because vertical wind speed seems to be ingeneral low in this cloud. You can also nicely see the speeding up of the spectra in the range spectrogram! Watch also the nice correlation between LWP maxima and MDV maxima!

Turbulence

At 400m the spectral peaks are still clearly separated and relatively narrow. Also the temporal variability of the MDV is relatively small.

A few range gates below, the separated spectra more and more merge to one peak due to turbulence broadening. The variations in MDV (see Time Spectrogram) are also much stronger and of larger „frequency“.

At 275m above ground, we can only identify one broad peak and the variations in MDV strongly increased.

2014-02-01, 11-12 UTC

Cloud Structure – Fallstreaks – Size Sorting Effects

Unlike the case before we find for this cloud tilted fallstreaks. At cloud top the wind speed is larger than at e.g. 1km. Can this explain the tilted fallstreaks?

Vertical profile of horizontal wind u (unidirectional)

Fallstreaks for:

Snowflake, vdop = 1 m/sRimed snow, vdop = 2 m/s

Note: The time for the particle to fall 600m (600s) is not neccesarily equal to the time one would derive from the fallstreak structure (300s) !

For more details, see my ERAD talk on Tuesday…

„Background“ ice/snow spectrum inbetween the fallstreaks

Moving to the left, we enter the left side of the streak where we expect the fastest particles to appear (note the right peak in the spectrum!)

Moving further to the left, the fastest particles disappear, and the fast peak seems to merge with the background spectrum

Super-cooled liquid water

RS indicates thick layer at cloud top with 100% RH. Within this layer we can often see the narrow liquid peak. LWP and plume structure seem to be correlated.

The HSRL lidar data show that liquid water already exists when the RS first reaches 100% (2km). However, the droplet sizes and/or concentrations seem to be low and thus the liquid peak is very weak.

Riming

At the top of the fallstreaks/plumes the particles are already (at least partly) rimed

But also outside the fallstreaks, the spectra reveal rimed particles below the thick liquid layer.

Multi-peak Spectra

It is not trivial to say where the multiple peaks come from since they are influenced by dynamical effects (e.g. merging of two fallstreaks) as well as by microphysical effects (e.g. riming).